If you accidentally deleted your calendars, reminders, bookmarks, or contacts from iCloud

You can go to iCloud.com and restore your missing calendars, reminders, bookmarks, or contacts from an earlier version.

When you restore from an earlier version, the version that you select will replace the existing calendars, reminders, bookmarks, or contacts on all your devices. Before you restore an earlier version, iCloud saves your current calendars, reminders, bookmarks, or contacts, so that if you want to reverse the restore, you can. Just repeat the steps to restore, and select the latest date to restore from.

If you recently made changes, you might lose some data.
You might want to create a backup before restoring.